On an off board flinging of emails, I got to thinking, this would be a good post for reference. For starters, I would like to know what ratio would work best for a 70 inch boat, 5.5 inches in diameter with a Speed 500 torque boat motor I have. Surely someone has come up with a rough formula for best runtime/performance for different size motors and different ratios. The brushless guys can jump in here too, but I think almost everyone here still uses brush motors. Different drive reductions would be interesting as well (I.E. belt drive vs planetary drive,etc). What works, what does not. Experiance is a big teacher! For myself, scale speed is fine, lotsa torque, I think. 70 plus inches with two engels is a big boat! Opinions? Ideas? Discuss!

In my case, sure. 1/96 scale. about 55 MM across the blades 70.4 inches long, 5.5 across. Boomer Speed 500 motor (there are different speed 500's this one is the torque motor, for boats) Weight- not sure, but dual Engels for ballast. That should get you close.

Okay, sounds like an Ohio? Forget about weight, it's wetted area and form that dictates the final velocity of the boat. Weight will affect acceleration and stopping however.

Ohio is officially 25 knots submerged. Yeah right, I think we both know it can do a heck of a lot more with 60000SHP when the Vanguard can do in excess of that with less than half that amount. So lets say we're aiming for a scale speed of 40 knots, we take the square route of the models scale, which works out at about ten, then divide 40 knots with that, which gives us about 4 knots for our model, or 4.4mph.

So 55mm prop, or almost 2.25" in old money, lets say the pitch of the prop is 1 times the diameter (standard for a Raboesch 7 blade scimitar). Assuming 25% prop slip. For 4.4MPH speed

The 500 motor I have is 12 volt #1788 with a 12,000 rpm on the box. I ordered a 4-1 belt drive last night, but can still change it. That 55 MM is in a pump jet. I want a long run time . Sthe o, another question would be a larger motor like the 700BB direct drive get you longer run times, all things the same. Hey, I was just thinking. I dont know who has tried it, but a 1/72 Thor Alfa with D&E 3.5 WTC in it has a 500 motor and 3-1, I think. I firewalled the throttle ONE TIME on the San Gabriel river on Gail's Alfa. That thing was so fast, it was almost impossible to hold an even depth after it got speed. It was wicked !!! That is when I learned to like running about scale speed. Although a little extra is nice.

Now I would think a 1/96 Akula with a 500 in it 3-1 should be fine (Close to the same dimensionally as the Alfa,or close enough). The Akula is the same frontal profile as what I am doing. Perhaps go to 3.5 to 1, for the extra mass, and "wet lenght" added. I could not be too far off?? As far as room for the belt drive, the one I am looking at is on Mike's Subworks website. It should fit, no problem. They are excellent vendors BTW... But so far this has only been for me. I would like to see this turn into a general answer spot for someone with the same questions.

For my ohio I tried several power set ups. I did a speed 500 (like yours) with a 3 to 1 and found it to be a bit over powered. Something this big doesn't need to travel that fast and it also gets harder to stop. Next I swapped the speed 500 for a pitman motor. This gave really nice slow speed ops but was lacking in crash back ability for me. So, I swapped out the gear reduction for a 2:1 drive. This seems to be the best set up for my boat. It runs for hours with no problems. Plenty of torques as well. Although, I did snap the 1/8" belt drive eventually and swapped it for a mo better 1/4" belt. Now mind you I am running 4.6 amp/hr of battery and have yet to get them bellow 50% capacity.

You mentioned the alfa that was uncontrollable due to speed. Take this into your speed factor consideration. With boomers the largest control surface isn't the stern planes, its the boat. As you go up and down the entire boat turns into a control surface and since they are so much longer they are even more effective. Even the flat turtle deck plays into this. If you make it to fast it will be a bit more difficult to control. These boats were not designed for speed but quiet. The fact that they are capable of x many knotts is not because they travel at those speed at all times but out of necessity for things like evading torpedos and escaping. In other words we almost never do it unless we have to. Even with the set up I have now I dialed the throttle limits on the radio to get the operation characteristics I require. Slow and stealthy.

Here is a thumb rule I learned (remember its a general one) for running direct drive. If you select a motor can size of similar diameter to the prop you are turning, you will normally be ok.

My observation on the Alfa was at "speed ridicules" faster than you can run, almost. So, I have seen a couple of answers. But Kevin, you went to a whole different motor because the 500 was overpowered at 3-1. Could dropping the ratio have corrected the problem? And, would you get a long run time with say, the 500 regeared,vs a larger motor, direct drive or reduction a little less? Interesting on the belt drive. Where did you get the unit?

I nearly forgot that the pitman is not the final configuration. These were my power set ups when I was running my rcabs cylinder. The reduction i was using was from mikes subworks ad was modified with parts from psme. Now this was all working great until I cam out with my MCS system. In an effort to keep my cylinder cost down all of my boats needed to ron on the same drive set up. Remember with the right reduction gear you can use most motors. My Ohio is no powered by 1 speed 400 motor. It is geared using a 4.66:1 planetary gear drive that is ball bearing supported. Even this little set up was more then enough power for the ohio and I currently have the power limits trimmed to below 50 %. I also still have rediculous run times.

Skip had some discussions while you were gone that discussed this whole concept. I would check rcu, rcgroups, and here for them. In it he said you could drive even the biggest of subs with a speed 400 if you used the correct reduction gear. I would agree with this. Especially considering I have done it.

Your motor should be plenty enough power. Most props have a posted designed operating rpm range. Picking a gear reduction should be some simple math given you already know the motors operating range. I would aim for a spot below the high end range though.

Not many bomber boats with propulsors/pumpjets. Vanguard is one, and a Vanguard at that scale isn't that long, so as you're a Russian sub man, I'm thinking this must be a Borei class?

Pumpjets tend to develop more thrust at lower RPM than a free running prop, so dependent upon pitch of the stator you may want to go lower in RPM still.

For instance the Prop Shop propulsor I have for my Vanguard needs only about 2000RPM to generate plenty of thrust, as the props pitch is very high. To push it beyond 2000RPM requires serious poke, I had a large Buhler motor drawing 60 watts to push it to 2500RPM.

Direct drive versus gearing? This is a much debated topic, with direct drive, you will usually need a physically larger motor to obtain the required torque, and the armature of the motor will have more windings (electrical gearing), which makes for more electrical resistance and thus lower efficiency.

With gearing, efficiency is lost through friction in the components. Belt or chain drive is the most efficient, about 95% transfer efficiency. Gears are less efficient, 90% at best, although can be as low as 75%. Most of this isn't worth worrying about for small models like ours, just pick a combination that works best for you.

Engel use this motor in Lafayette and 212 kits, and it powers both very well. The Akula is a much fatter hull than the Lafayette, so expect slightly less speed.

The 720BB (not 700BB, that's a different animal) is a low revving high torque motor that puts out about 36 watts of power, and runs at about 4100RPM on 12 volts. It can swing up to 60mm props direct drive, perhaps larger dependent on pitch, beyond that you need to gear it down. If it's well matched to the prop, it should draw about 36 watts under full load.

Interesting answers. Yes, it will be a Borie... Sounds like the 500 I have I might should go with the 4-1 after all? I went back last night and changed to a 3.5-1. But still not to late. Although Mike may thing I am going scitzoid on him! The motor you showed on the Engel site is a ballast motor. But the specs are interesting. I had a devil of a time finding the 500 I did. In fact, I bought 2, because they are getting so hard to find. Seemed, at the moment, everyone was thinking brushless motors. Kevin, the wider belt sounds good. Where did you get it? So, opinions. I will settle today on mine. 3.5-1, 4-1?

The Engel motor is just a high wind low RPM 500 size motor that is used both for propulsion, and is also used on some of their piston tanks. You could probably find a similar motor elsewhere too.

It's difficult to say exactly what RPM you should go for, as I don't know the pitch of the prop you're going to be using, but I would say somewhere between 2-3000RPM sounds about right to me as per earlier posting.

BTW if you want to see something tangible have a look at this video and build thread of an OTW Vanguard. The boat is set to half throttle in that clip, as the builder is still ironing out the bugs in the boat, but already running at about scale speed. The propulsor is a Prop Shop version using Dave Merrimans masters which he made up for the OTW Vanguard build he did a few years back. This is lower in pitch than the older propulsor that used to be supplied with the kit.

The Vanguard is slightly smaller than a Borei, but not much and the propulsors are pretty much identical diameter. This boat uses a 12 volt speed 500 geared through a planetary gearbox with 5:1 ratio putting out about 3000RPM and plenty of torque.